Approximately 5 to 10% of the population suffer from impaired hearing. The degree of hearing loss ranges from mild, to moderate, to severe, to profound hearing losses on the verge of deafness, and finally, to acquired or congenital deafness. The cause for such hearing losses can lie in the region of the ear which conducts the sound wave (ear drum, middle ear), in the inner ear (cochlea), or in the auditory nerve or central auditory processing. Depending upon the cause, site, and degree of hearing difficulty, operative therapy, rehabilitation, drug therapy, or other therapies may be indicated. When these therapies are insufficient or unsuccessful, there are a variety of technical hearing aids (auditory prosthesis) available in order to improve hearing.
In the prior art, hearing aids have been based on one of two basically different principles: acoustic mechanical stimulation, or electrical stimulation. With acoustic mechanical stimulation, sound is amplified in various ways and delivered to the inner ear as mechanical energy. This may be through the column of air to the ear drum, or direct delivery to the ossicles of the middle ear. Acoustic mechanical stimulation requires that the structure of the cochlea, hair cells, and auditory nerve all be intact. The more hair cells which are destroyed or not functioning properly, the less effective is acoustic mechanical stimulation.
Electrical stimulation functions very differently. With this method, used when the structures of the cochlea-in particular, the hair cells--are disrupted, the sound wave is transformed into an electrical signal. The resulting stimulation pattern can be "understood" by the auditory nerve. Electrical stimulation does not require that the structure of the cochlea and the hair cells be intact. It is only necessary that the auditory nerve, as well as the central processing centers, are sufficiently intact. With electrical stimulation, the stimulating electrodes should be placed as close as possible to the nerve endings of the auditory nerve. This occurs optimally when an electrode carrier is inserted into the cochlea. As a rule, this procedure results in destruction of the structures of the inner ear which may still be functioning. Therefore, this technique is not used when there is significant residual hearing.